1. Field of the Invention
This invention relates to a method for making 2,6-naphthalene dicarboxylic acid by the oxidation of 2,6-diisopropylnaphthalene or its oxidation derivative with an oxygen-containing gas under liquid-phase conditions and in the presence of a catalyst comprising metal and bromine components.
2. Discussion of the Prior Art
Fibers and films produced from polyethylenenaphthalate have improved strength and thermal properties relative to fibers and films produced from polyethyleneterephthalate and are especially useful in applications such as tire cords, magnetic tape backings and hot-fill containers. However, the use of 2,6-naphthalene dicarboxylic acid is inhibited by its relatively high cost, which is due to the relative unavailability and high cost of the preferred feedstock, 2,6-dimethylnaphthalene, which can be readily oxidized to 2,6-naphthalene dicarboxylic acid under conditions that are conventional for the oxidation of alkylated aromatics--that is, under liquid phase conditions, in a solvent, at elevated temperature and pressure, by an oxygen-containing gas, and in the presence of a catalyst comprising cobalt, manganese and bromine components.
It has been proposed to substitute the more readily available and less expensive 2,6-diisopropylnaphthalene or its oxidation derivative for 2,6-dimethylnaphthalene or its oxidation derivative in the aforesaid conventional oxidation. For example, Hirose et al. (Teijin Petrochemical Industries Ltd.), published European Patent Application No. 142719, disclose that the use of the aforesaid conventional oxidation conditions for the oxidation of 2,6-diisopropylnaphthalene, or its oxidation derivative, results in an extremely low yield of 2,6-naphthalene dicarboxylic acid, which is also of low purity because of the formation of relatively large amounts of undesirable by-products. This published European Patent Application also discloses that variation of the aforesaid conventional oxidation, either by the use therein of a plurality of oxidation stages in which the reaction temperature is increased stepwise or continuously from a relatively low temperature in an early stage to relatively higher temperatures in a latter stage, or by maintaining a low concentration of 2,6-diisopropylnaphthalene or its oxidation derivative in the solvent in the oxidation, did not afford acceptable yields of 2,6-naphthalendicarboxylic acid.
By contrast, the method of the invention disclosed and claimed in European Patent Application No. 142719 employs a relatively large amount of the aforesaid oxidation catalyst containing a heavy metal element selected from the group consisting of cobalt and manganese and a bromine element, with 0.2 to 10 gram-atoms of the heavy metal element being used per mole of 2,6-diisopropylnaphthalene or its oxidation derivative.
Furthermore, published Japanese patent application Kokai No. 120342/87 discloses a process for oxidizing 2,6-diisopropylnaphthalene or its oxidation derivative to 2,6-naphthalene dicarboxylic acid in a reaction medium containing at least 50 weight percent of propionic acid and in the presence of a catalyst comprising (1) a bromine element, (2) cobalt or manganese or a mixture thereof, and (3) an alkali metal element.
Published Japanese patent application Kokai No. 120343/87 discloses a process that is very similar to that of published Japanese patent application Kokai No. 120342/87 but in which the solvent contains at least 50 weight percent of at least one monocarboxylic acid selected from butyric acid, valeric acid and benzoic acid.
Published Japanese patent application Kokai No. 246143/86 discloses a process that is very similar to that of published Japanese patent application Kokai No. 120342/87 except that at least 70 weight percent of the solvent is acetic acid or propionic acid or a mixture thereof and except that 1.1 to 15 gram atoms of the alkali metal component of the catalyst must be employed per gram atom of the bromine component of the catalyst.